Vaccination offers protection against future exposure to pathogens because it

Because it primes the immune system for future exposures to virulent pathogen. Vaccination introduced attenuated (non-virulent) form of a pathogen into the body and allows the immune system to develop antibodies against the antigen. In case of future real exposure to the pathogen, the immune system will already have developed memory B and t cells against the pathogen hence will have an effective response.  

you must remain stopped until the stop signal is retracted and the red lights are turned off. approaching traffic in the opposite roadway of a divided highway shall not be required to stop, even if the school bus has the stop-arm extended and the alternately flashing warning signal lamps on.

mammal

explanation: